The invention relates to anti-skinning agents containing mixtures of organic compounds (combinations of additives) and coating compositions containing them. The organic compounds are selected from the groups of organic hydroxylamines, and xcex2-dicarbonyl compounds and/or selected derivatives of formic acid. The invention further relates to compositions containing these anti-skinning agents, like coating compositions such as oxidatively drying paints.
Colorless and pigmented oxidatively drying paints and coatings based on oxidatively drying oils, alkyd resins, epoxy esters and other oxidatively drying refined oils are known. These oils and binders crosslink oxidatively under the influence of oxygen (preferably atmospheric oxygen) by means of the addition of driers, such as metal carboxylates of transition metals. If this crosslinking takes place before the product is actually used, they can form a solid binder film, a skin, on the surface when stored in open or closed containers. This is highly undesirable and should therefore be avoided since it makes the paint more difficult to work with, and commonly interferes with the uniform distribution of the driers. The accumulation of the driers in the paint skin that forms can lead to considerable delays in the drying of the paint when it is applied.
Skinning in the paint film after application is also disadvantageous. Excessively rapid drying of the surface of the paint prevents the lower film layers from drying evenly because they are shielded from oxygen, which is prevented from sufficiently penetrating into and dispersing within the paint film. This can lead among other things to flow problems in the paint film, adhesion problems, or insufficiently hard films.
It is known to add organic substances to the paint that inhibit the reaction of the drier metal with (atmospheric) oxygen by binding the oxygen or by complexing of the drier metal. Formulations of known compounds that are suitable for this purpose can be found e.g. in H. Kittel xe2x80x9cLehrbuch der Lacke und Beschichtungenxe2x80x9d, Colomb Verlag 1976; J. Bieleman xe2x80x9cLackadditivexe2x80x9d Wiley VCH 1998 or Rxc3x6mpp Lexikon xe2x80x9cLacke und Druckfarbenxe2x80x9d, Thieme Verlag 1998.
Furthermore, DE-A 3 418 988 describes the use of aliphatic xcex1-hydroxy ketones as anti-skinning agents, and DE-A 1 519 103 discloses N,N-dialkylated hydroxylamines for this purpose. Because of their low volatility, however, these agents can lead to severe delays in drying and often also to reduced film hardness values, so that their possible applications are limited. They have not been able to gain acceptance as anti-skinning agents.
EP-A 0 903 380 describes the use of xcex1-keto-functionalised carboxylic acids and dicarboxylic acids and of derivatives thereof, optionally in combination with diketo compounds, as anti-skinning agents. A disadvantageous property of such products lies in their limited stability and hence restricted storage life.
Finally, pyrazole or imidazole derivatives, optionally in combination with aliphatic diketones, in particular acetylacetone, are recommended for this purpose in WO 00/11090. However, the cited aromatic amines can cause severe discoloration in the coating composition and also in the applied paint film and likewise give rise to severe delays in drying.
Oximes (in particular butanone oxime) or suitable phenolic compounds are mostly used today as anti-skinning agents in industry. The phenolic anti-skinning agents display a significant delay in surface drying, however, such that alone they are only suitable for certain coating compositions. Oximes such as e.g. methyl ethyl ketoxime or butyraldoxime, on the other hand, display only slight delays in surface drying due to their volatility. The most significant disadvantage of the oximes, which are widely used today, lies in their toxicity. In long-term inhalation studies on rats and mice, for example, an increased occurrence of liver tumors was observed after exposure to butanone oxime, as a result of which the German MAK committee has classified the substance as a category III A 2 carcinogen (MAK list 1997) [MAK=maximum allowable concentration in the workplace]. As a consequence of this, users have to observe elaborate personal protection precautions when working with paints containing oximes as anti-skinning agents.
It is an object of the present invention to develop organic compounds which prevent skinning in paints and coating compositions over an extended period and display only a slight delay in surface drying. Furthermore, the film hardness values obtained in the resulting films or coatings should not be negatively influenced and no discoloration should be caused. The products should display no disadvantageous toxicological properties.
It is another object of the invention that the organic compounds can be incorporated into many different oxidatively drying paints and coatings and by virtue of their physical properties can be used straight-forwardly and for many purposes in corresponding coating compositions.
Surprisingly it was found that these requirements could be achieved by the use of mixtures containing the organic compounds described below. In particular, the above-mentioned disadvantages of the specified hydroxylamines as anti-skinning agents could also be avoided by combining such substances with the additional compounds described below, and hence products that better satisfy requirements as anti-skinning agents are obtained.
In air-drying paints the mixtures according to the invention are suitable for preventing undesirable skinning, for improving the complete drying of paint films after application and for reducing the tendency of the applied paint film to discolor.
The invention relates to an anti-skinning agent containing
a) an organic compound of formula (I) 
where
R1 and R2 mutually independently represent hydrogen, a linear or branched, saturated or unsaturated C1-C20 aliphatic radical, which can optionally be mono- or polysubstituted, or a C6-C12 aryl radical, a C7-C14 araliphatic radical or a C5-C7 cycloaliphatic radical,
and either or both of
b) an organic compound of formula (II) 
where
R3 represents hydrogen, a hydroxyl radical, a substituted or unsubstituted, saturated or unsaturated C1-C19 aliphatic radical, a C1-C7 alkoxy radical, or an amine radical xe2x80x94NR7R8,
R4 represents hydrogen, hydroxyl radical, a substituted or unsubstituted, saturated or unsaturated C1-C19 aliphatic radical, a C1-C7 alkoxy radical, or an amine radical xe2x80x94NR9R10,
R7 and R8 mutually independently represent hydrogen or a C1-C6 aliphatic radical,
R9 and R10 mutually independently hydrogen or a C1-C6 aliphatic radical, and
c) an organic compound of formula (III) 
where
R5 represents hydrogen, a hydroxyl radical or a C1-C7 alkoxy radical and
R6 represents hydrogen or an amine radical xe2x80x94NR11R12,
R11 and R12 mutually independently represent hydrogen, substituted or unsubstituted C1-C6 aliphatic radical, or substituted or unsubstituted C5-C7 cycloaliphatic radical, or amine radical xe2x80x94NR1R2 
with the proviso that R5 and R6 are not simultaneously hydrogen.
The invention also relates to compositions of matter containing these anti-skinning agents.
For the purposes of the invention mixtures of organic compounds of formulae (I) to (III) are used alone or as solutions or dispersions or emulsions in water and/or organic solvents. Suitable organic solvents include all conventional solvents, such as aromatics, white spirits, ketones, alcohols, ethers and fatty acid esters.
For the use according to the invention the organic compounds of formulae (I) to (III) can be used in a broad range of mixtures with one another. They are preferably used in the ratio (I):(II) or (III)=0.1:10 to 10:0.1 parts. In a mixture consisting of all three components, each of the components can mutually independently preferably be used in the ratio 0.1 to 10 to each of the other components used. They can be used in pure form or in aqueous solution or aqueous dispersion or emulsion or in the form of solutions in organic solvents, whereby aqueous in this context is intended to mean that water is either the sole solvent or is added in a quantity of over 50 wt. % relative to the solvent blend together with conventional organic solvents (e.g. alcohols).
The amount of anti-skinning agents primarily depends on the content of binder and drier used in the particular coating composition. As a general rule between 0.001 and 2.0 wt. % of mixtures of compounds according to formulae (I) to (III) should be added. Preferred amounts to be used are 0.01 to 0.5 wt. %, relative in each case to the overall recipe of the coating composition. The amounts can also depend on the type of binder and the pigments used. Thus in special systems the relative amount of additive to be used can also be greater than 2.0 wt. % (relative to the overall recipe).
It is an advantage of the anti-skinning agent of the present invention that it reliably prevent skinning in a wide range of binders and when used with various driers but does not unfavorably influence other drying properties of the paint.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.